What was another clever trick Bonhoeffer played on a friend?

Another time, a friend from Bonhoeffer’s church in England asked him to play a song with him on the piano. Bonhoeffer tricked his friend by playing the notes in the wrong key.

The notes almost sounded like the right ones. Halfway through the song, his friend realized the trick. Bonhoeffer laughed aloud—and then played the right notes.